The invention is related to applications bearing Ser. No. 414,060 entitled "PRESSURE MEASURING SYSTEM" which was filed November, 1973 now U.S. Pat. No. 3,895,527, and Ser. No. 414,062 entitled "METHOD AND APPARATUS FOR MEASURING PRESSURE RELATED PARAMETERS" which was filed November, 1973 now U.S. Pat. No. 3,898,877.
The invention relates to the measurement of fluid pressure at a remote location and is of particular application to measurement of pressures in a borehole.
The pressures of fluid in boreholes are particularly important in the production of oil and gas. Secondary recovery operations, for example, require pressure information to determine a number of factors necessary to the success of the operations.
Preliminary to the secondary recovery operations borehole pressures give an indication of well productivity potential and the amount of fluid that will be required to "fill up" the space in the formation before oil and gas will begin to be forced out. During operations the measurement of pressure changes in a number of boreholes in a formation indicate the location of injection fluid flood fronts as well as the efficiency with which the flood front is sweeping through the formation.
In addition to secondary recovery operations, borehole pressures are important in other areas of oil and gas production. For example, pressure measurements may be used to indicate wellbore damage or any number of other problems in pumping wells.
The invention provides substantial improvement over previous methods of borehole pressure measurement. Some previous methods provided only periodic measurements, which are not only inconvenient and time consuming due to the necessity of inserting instrumentation into the borehole at each measurement but also incomplete in the representation of borehole conditions. An example of this type of system is disclosed in U.S. Pat. No. 3,712,129 to Rhoades. Each time a pressure measurement is desired, Rhoades charges an open-ended tube with a gas until the gas bubbles from the bottom of the tube. The pressure in the tube at the surface at which bubbling begins is the pressure of the borehole fluids. In contrast the present invention is capable of providing continuous pressure information.
Other bubble tube systems using the principle of pressure equalization include U.S. Pat. No. 1,289,755 to Haynes, which relates to measuring the depth of water. The Haynes apparatus uses a tube having an enlarged sleeve on its lower end. The tube and sleeve containing air at atmospheric pressure are lowered into the water. Shortly after the tube enters the water, a pressure storage tank begins to automatically supply pressurized air to the tube through a spring-loaded valve. The spring tension is initially overcome by the sleeve, which collects a larger volume of air and has a larger cross-sectional area than the tube. Since force equals pressure times area, the larger area of the sleeve cross-section allows the air therein to be forced into the tube at a lower water pressure. This permits the valve to be opened at a shallower water depth measurement to thereby begin.
As the tube is lowered into the water, the air therein is compressed by the force of water pressure, which delays the opening of the valve. In order to make the valve opening closely follow movements of the lower end of the tube, the compression must be compensated. This can be done by increasing the volume of air in the tube. This extra volume of air is provided by the sleeve. The sleeve is used only when the tube is initially placed in the water and plays no part thereafter. It does not prevent water from entering the upper part of the tube, nor does it act to maintain the water level near the bottom of the tube in order that pressures measured closely reflect those at the bottom thereof. The pressures are measured at different depths at the pressure required to bubble the air from the bottom of the tube, not at a pressure at which the water level is higher in the tube than the bottom thereof.
A number of other pressure measuring devices have been devised to overcome the problems inherent in periodic measurement by providing for permanent installation in producing wells. One such device operates with a downhole pressure transducer having an electronic scanning system for converting downhole pressure into the data transmittable to the surface on a conductor cable. The cable is normally attached to the outside of the tubing and the transducer is mounted on the lower end of the tubing string. The electronics in such a system is expensive and produces maintenance problems stemming in part from high temperatures and corrosive fluids often present in boreholes. In addition, an electronic system using scanners and transmitting such data over conductors is subject to problems of maintaining a high resolution, and thus data may not be as accurate as needed to determine changes in reservoir conditions. In addition, downhole pressure transducers are often intricate in design and thus are subject to the hostile pressure, temperature and chemical fluids environment of wellbores.
It is therefore an object of the present invention to provide a new and improved apparatus for detecting a borehole pressure.